While randomized controlled trials (RCT) support early PN in premature infants and late PN in critically ill adults, the optimal time to begin PN is unknown for critically ill infants and children. Acute malnutrition may develop within 48 hours of admission in critically ill infants and children, and repleted energy stores are predictive of survival. And yet, due to concerns for PN-associated infectious morbidity, current PICU standard of care is to supplement with PN only in children who fail to enterally feed, as late as 7 days into their admission. Delays in nutrition may have long-term effects on cognitive outcome in older infants and children. In premature infants, PN begun within hours of birth results in improved 18-month neurocognitive outcome without an increase in infectious complications. An RCT is needed to determine if early PN in critically ill infants and children prevents acute malnutrition and improves short and long-term outcomes of PICU hospitalization.